Whanganui recorded the warmest temperature in the country on Sunday with a high of 20.2C.
But a new set of cold fronts is making its way from the South Island, bringing a cold southwest change and cooler air to the region.
Metservice meteorologist Mmathapelo Makgabutlane said there had been dynamic atmosphere over the Tasman Sea which had seen several fronts and troughs pass over the upper half of the country.
"There were plenty of showers to go around on Monday as a trough passed with an unstable northwest flow and some thundery weather in the western and central North Island," she said.
"The wind change to southwesterly is now bringing cold fronts with cooler daytime temperatures for Whanganui."